1. Field of the Invention
The present invention relates to processing and storage of compressed visual data, and in particular the processing and storage of compressed visual data for pausing and resuming transmission of an MPEG data stream.
2. Background Art
It has become common practice to compress audio/visual data in order to reduce the capacity and bandwidth requirements for storage and transmission. One of the most popular audio/video compression techniques is MPEG. MPEG is an acronym for the Moving Picture Experts Group, which was set up by the International Standards Organization (ISO) to work on compression. MPEG provides a number of different variations (MPEG-1, MPEG-2, etc.) to suit different bandwidth and quality constraints. MPEG-2, for example, is especially suited to the storage and transmission of broadcast quality television programs.
For the video data, MPEG provides a high degree of compression (up to 200:1) by encoding 8×8 blocks of pixels into a set of discrete cosine transform (DCT) coefficients, quantizing and encoding the coefficients, and using motion compensation techniques to encode most video frames as predictions from or between other frames. In particular, the encoded MPEG video stream is comprised of a series of groups of pictures (GOPs), and each GOP begins with an independently encoded (intra) I frame and may include one or more following P frames and B frames. Each I frame can be decoded without information from any preceding and/or following frame. Decoding of a P frame requires information from a preceding frame in the GOP. Decoding of a B frame requires information from both a preceding and a following frame in the GOP. To minimize decoder buffer requirements, transmission orders differ from presentation orders for some frames, so that all the information of the other frames required for decoding a B frame will arrive at the decoder before the B frame.
A GOP can be “open” or “closed.” A GOP is closed if no prediction is allowed from any frame in a previous GOP. In other words, there are no B or P frames that require any information outside the GOP for decoding. A GOP is open if prediction is allowed from a frame in a previous GOP. In other words, there is a B or P frame that requires information in a frame outside of the GOP for decoding. In the typical case of an open GOP, the transmission order of the GOP begins with an I frame and has at least one B frame following the I frame. In the presentation order, this B frame precedes the first I frame in the GOP, and this B frame requires, for decoding, the last frame of a preceding GOP.
In addition to the motion compensation techniques for video compression, the MPEG standard provides a generic framework for combining one or more elementary streams of digital video and audio, as well as system data, into single or multiple program transport streams (TS) which are suitable for storage or transmission. The system data includes information about synchronization, random access, management of buffers to prevent overflow and underflow, and time stamps for video frames and audio packetized elementary stream packets embedded in video and audio elementary streams as well as program description, conditional access and network related information carried in other independent elementary streams. The standard specifies the organization of the elementary streams and the transport streams, and imposes constraints to enable synchronized decoding from the audio and video decoding buffers under various conditions.
The MPEG-2 standard is documented in ISO/IEC International Standard (1S) 13818-1, “Information Technology-Generic Coding of Moving Pictures and Associated Audio Information: Systems,” ISO/IEC IS 13818-2, “Information Technology-Generic Coding of Moving Pictures and Associated Audio Information: Video,” and ISO/IEC IS 13818-3, “Information Technology-Generic Coding of Moving Pictures and Associated Audio Information: Audio,” which are incorporated herein by reference. A concise introduction to MPEG is given in “A Guide to MPEG Fundamentals and Protocol Analysis (Including DVB and ATSC),” Tektronix Inc., 1997, incorporated herein by reference.
One application of MPEG-2 coded video is video-on-demand (VOD). In a VOD application, a server streams MPEG-2 coded video in real time to a subscriber's decoder. The subscriber may operate a remote control providing well-known video cassette recorder (VCR) functions including play, stop, fast-forward, fast-reverse, and pause. In a typical implementation of the pause function, the server responds to a pause command from the subscriber by simply stopping transmission of the MPEG-2 coded video stream. Then the decoder loses synchronization, and the subscriber sees Oa result that is dependent on the performance of the decoder. The server responds to a resume command from the subscriber by resuming transmission of the MPEG-2 coded video stream. The decoder fails to present some of the video frames while re-synchronizing to the video stream. Another method that has been used is to pause by sending padding packets with continuous PCR stamps that will keep the decoder synchronized so that when the pause ends the decoder will not reset. This will create artifacts for a number of frames if the GOP is open.